Maybe because guns are not the cause of mass killings.

The purpose of the Second Amendment was not to arm the military.

The second amendment was intended as a check and balance to prevent a rogue military from taking over the government.

The armed population was supposed to outnumber any military takeover of the government.

The gun by itself does not kill anyone.

Some human being has to handle and use the gun to make it kill someone.

It is a very rare event when a gun somehow goes off by itself and kills someone.

There are several cases where a criminal has grabbed a gun in an officer's hand and tried to take it from the officer. When the gun is pulled, the officer's finger pulls the trigger, killing the criminal.

Guns are used in killings because guns are handy.

It is probably easier to get a gun than it is to get any other killing device.

The angered killer usually grabs the handiest thing he can find.

Guns are used in killings because guns can kill at a distance.

This is not usually the reason a gun was chosen as the weapon. But it does help to conceal the shooter.

Most shooters carry the gun right into the place where the shooting occurs, often concealed by clothing or a carried object.

Guns laws don't work because criminals do not obey laws.

Most criminals find illegal ways around all gun control laws.

FBI records show that most of the guns used in crimes and killings were stolen or obtained illegally.

Often the killer used the victim's own gun,

Many shooters take the legal gun of a relative.

Gun laws don't keep guns out of criminal hands.

Those who want guns for wrong purposes can steal them.

Those who want guns can make them.
 - The materials needed can be found in hardware and grocery stores.
 - This includes materials to make propellants for projectiles.
 - I used to have a modelmaking lathe and mill. With those, I now realize that I had most of the tools needed to make a gun.
 - Many crude guns are made of pipe.
 - Look up the term "zip gun".

The want to keep guns out of the "wrong hands". But which hands are the wrong hands?

Other than age and criminal record, there is no real determiner of who should not have weapons.

They must be careful to not discriminate against any legitimate political belief.

Often bullying at school grows into a terrorist mentality.

Gang members definitely should not have firearms.

Many shooters have never shown any violent tendencies until they committed their one terrorist act.

Some shooters have valid grievances against an employer or a government.

Most of the guns in the US are never used in any crime or killing.

FBI records show that 99.8% of all privately held guns in the US have never been used in any crime or killing.

This means that the remaining 0.2% were used in all of the crimes and killings.

FBI records show that in many cases, the same gun was used in multiple crimes

FBI records show that most of the guns used in crimes and killings were stolen or obtained illegally.

When guns are unavailable, other weapons are used instead.

The most common weapons other than guns that are used are:
 - Archery, knives, bombs, poisons, clubs, spears, vehicles, swords, falls from heights, drowning,
  suffocation, choking, and heavy missiles.

More esoteric weapons include:
 - Boat anchors, sailboat booms, golf clubs, hockey sticks, bowling balls, pitons, sledgehammers,
  plastic bags, scissors, pool cues and balls, pool skimmers, pillows, farm tools, wires, electricity,
  animals, drones, golf carts, bicycles, skateboards, beer bottles, boomerangs, air compressors,
  slingshots, bathtubs, and many others.

Mass killings occur because people no longer value life.

These people do not care who they harm. They want their own way at any cost.

Public schools no longer teach the value of human life.

Many movies made today treat human life as expendable.

Those who want abortion to be legal do not value human life.

Teachers do not teach the value of life because they favor abortion.

Much of the material taught in schools treats life as being expendable.

Children are more likely to not value human life than adults.

satan wants people to not value human life because he wants the souls of the dead.

Raising the legal age to have a handgun to 21 means that fewer people who do not value life will have guns.

Mass killings happen because satan is trying to take over.

When satan possesses someone, that person is often mad at the entire world.

During a mass killing, satan gets all of the unsaved souls of the dead.

satan also triggers all of the copycat mass killing events.

In events prompted by satan, the killer often commits suicide.

Often the killer obeying satan spews a hate-filled diatribe against Jesus Christ just before the killing spree.

satan wants gun control rather than teaching the value of life.

Government is aiding satan by supporting the works of satan (anything that disobeys God):

All forms of sexual immorality, including adultery, unmarried sex, and everything GLBT

Abortion, euthanasia, and some forms of birth control

High taxes and too-big government

Despotic governments remove the power of people to protect themselves to preserve their regimes.

Government not enforcing the law, letting crime run rampant

Notice how those supporting the position of satan are the ones committing all of the violence.